1. Field of the Invention
This invention relates to a rubber composition for a tire tread developing an excellent gripping performance as well as a pneumatic tire using the same.
2. Description of Related Art
Recently, it is strongly demanded to provide pneumatic tires having higher running performances accompanied with improvement of automobile performances, pavement of roads and development of expressway networks. As the running performances of the tire become higher, the automobile can be run accurately and safely at a higher speed. Particularly, the gripping performance exemplified by acceleration performance and braking performance is an important requirement in the tire.
In order to obtain a high gripping performance, there has hitherto been proposed a method wherein styrene-butadiene copolymer rubber having a high bound styrene content as a rubber having a high glass transition temperature was used in the rubber composition for the tire tread.
According to this method, however, the improvement of the gripping performance is obtained, but there is caused a problem that the value of tan .theta. lowers with the rise of rubber temperature during the running of the tire and hence the gripping performance rapidly lowers.
In order to prevent the degradation of the gripping performance due to the temperature rise, there is proposed a technique of using a copolymer rubber obtained by copolymerizing a monomer such as 1,3-butadiene, styrene, isoprene or the like with a methacrylate compound or acrylate compound containing diphenyl phosphate group such as diphenyl-2-methacryloyloxyethyl phosphate, diphenyl-2-acryloyloxyethyl phosphate or the like (see JP-A-59-187011). However, this technique can not be applied to natural rubber, but also undesirably damages properties inherent to a polymer such as styrene-butadiene copolymer rubber or polybutadiene rubber in accordance with the production conditions.
On the other hand, there is a method of increasing the value of tan .theta. in the rubber composition by using a compounding system filled with great amounts of process oil and carbon black. According to this method, the gripping performance is improved, but the fracture properties and wear resistance are considerably degraded, so that the filling amount is critical and hence it is difficult to render the gripping performance into the required high level.
There is another method wherein particular amount of particular imidazole compound or imidazoline compound and particular proton acid derivative are compounded with the rubber ingredient to increase the value of tan .theta. and the resulting rubber composition is used in the tire tread to improve the gripping performance (see JP-A-63-139931). In this method, the antiscorching property is somewhat improved as compared with a method described in JP-A-63-10645, but does not yet arrive at a practical level.